Method of forming nozzles

ABSTRACT

A nozzle in a nozzle plate for an inkjet printhead is formed by directing a laser beam at a nozzle plate. Accurate control of the divergence of the beam is achieved by splitting the beam into sub-beams, each sub-beam having divergence with an origin lying apart from the point at which the beam is created by splitting, and thereafter recombining the sub-beams. Greater accuracy in the taper and inlet shape of the manufactured nozzle is thereby obtained.

The present invention relates to methods and apparatus for forming anozzle in a nozzle plate for an ink jet printhead, the nozzle having anozzle inlet and a nozzle outlet in respective opposite faces of saidnozzle plate.

WO93/15911 concerns methods of forming nozzles In a nozzle plate for aninkjet printhead utilising a high energy beam, in particular theablation of nozzles in a polymer nozzle plate using an excimer laser. Bymeans of a mask having a single aperture, a high energy beam is shapedprior to being directed by a converging lens onto the surface of anozzle plate where a nozzle is formed.

WO93/15911 recommends increasing the divergence of the beam incidentinto the aperture of the mask by passing the beam through a layer suchas a ground or etched surface or a thin film containing a medium havingsuitable light-scattering properties such as a colloid or opalescentmaterial. Such a layer may be placed against a convergent lens which isitself located upstream of the mask for the purpose of focusing the beaminto the aperture.

The divergence of the beam will determine the angle of taper of thenozzle. Furthermore, a second mask can be used to reduce the angle ofdivergence in one plane of the beam relative to another (both planescontaining the beam axis), thereby to obtain a nozzle having a greaternozzle taper in one plane than in another. This will result in a nozzleinlet that is larger in one direction than in another directionperpendicular thereto—WO93/15911 points out that this advantageouslyallows the nozzle ink inlet to match the (generally rectangular) shapeof an ink channel in the printhead with which the nozzle willcommunicate, whilst allowing the nozzle outlet to remain preferablycircular.

The present invention has as its objective improvements to the processesdescribed in the aforementioned WO93/15911, in particular to the mannerin which the nozzle taper and the shape of the nozzle inlet and outletare controlled.

According to a first aspect, the present invention comprises the methodof forming a nozzle in a nozzle plate for an ink Jet printhead, thenozzle having a nozzle inlet and a nozzle outlet in respective oppositefaces of said nozzle plate, the method comprising the steps of:

-   -   directing a high energy beam towards said nozzle plate;        introducing divergence into said beam; thereafter directing said        beam at a single aperture of a mask, thereby to shape said beam;        and thereafter passing said beam through beam converging means        prior to impingement on the face of said nozzle plate in which        said nozzle outlet is formed, thereby to form a nozzle, the        nozzle outlet being conjugate through said beam converging means        with said single aperture; wherein    -   the step of introducing divergence into said beam comprises        splitting said beam into a number of sub-beams, each sub-beam        having divergence, the origin of divergence of each sub-beam        lying apart from the point at which the respective sub-beam is        created by splitting; the sub-beams thereafter being passed        through further beam converging means prior to being recombined        and directed through said single aperture of a mask, wherein the        dimensions of the section of said recombined beam directly prior        to impinging the plane of said mask are substantially equal to        the dimensions of the aperture in said mask.

The present invention includes the step of introducing divergence intosaid beam by splitting said beam into a number of sub-beams, eachsub-beam having divergence, the origin of divergence of each sub-beamlying apart from the point at which the respective sub-beam is createdby splitting, the sub-beams thereafter being passed through further beamconverging means prior to being recombined. This arrangement allowssubstantially more accurate control of the angle of divergence of thebeam than has been possible in prior art arrangements: as mentioned,WO93/15911 proposes increasing the divergence of the high energy beam byscattering the light using a ground or etched surface or a thin filmcontaining a medium having suitable light-scattering properties. It hasbeen recognised in the present invention that divergence can be obtainedin a much more controlled manner by splitting the high energy beam intoa number of sub-beams which are subsequently recombined. Furthermore,the beam is split such that each sub-beam has divergence having anorigin at a point lying apart from the point at which the respectivesub-beam is created by splitting. It will be appreciated that thedivergence obtained in this manner—which may be achieved using a lens tocreate each sub-beam—will be subject to substantially less variationthan is achieved using prior art methods based on scattering. It followsthat less variation in the angle of divergence of the combined beam willresult in less variation in the angle of taper of the manufacturednozzles—resulting in better ink ejection performance of the final inkjetprinthead.

Furthermore, by directing the recombined beam through a single apertureof a mask, with the dimensions of the section of the recombined beamdirectly prior to impinging the plane of said mask being substantiallyequal to the dimensions of the aperture in said mask, the high energybeam that finally impinges on the nozzle plate to form a nozzle does nothave its divergence reduced by any significant amount by the mask.Consequently, the full range of beam divergence is available to formnozzle bores having a correspondingly high taper angle from outlet toinlet.

According to a second aspect, the invention comprises the method offorming a nozzle in a nozzle plate for an ink jet printhead, the nozzlehaving a nozzle inlet and a nozzle outlet in respective opposite facesof said nozzle plate, the method comprising the steps of:

-   -   directing a high energy beam towards said nozzle plate;        introducing divergence into said beam; and thereafter passing        said beam through beam converging means prior to impingement on        said nozzle plate, thereby to form a nozzle; wherein    -   the step of introducing divergence into said beam comprises        passing said beam through an array of optical elements to create        an array of sub-beams, each sub-beam having divergence, the        origin of the divergence of each sub-beam lying apart from the        respective optical element; said array of sub-beams being        thereafter directed towards first reflecting means for        reflecting towards second reflecting means, said second        reflecting means reflecting towards said nozzle plate; the        positional relationship of said first and second reflecting        means being such that a parallel beam impinging on said first        reflecting means is reflected frbm said second reflecting means        as a converging beam; the arrangement of said optical elements        being such that all incoming sub-beams are directed by said        first reflecting means towards said second reflecting means,        thereafter to impinge on said nozzle plate.

This second aspect of the invention also utilises the concept ofsplitting (by means of an array of optical elements) a high energy beaminto sub-beams having an origin of divergence lying apart from the planeof beam splitting and thereafter recombining the sub-beams through beamconverging means. It therefore shares with the first aspect of theinvention the advantage that the resulting angle of the beam can beaccurately controlled.

In addition, the high energy beam is directed at the nozzle plate bymeans of first and second reflecting means and the optical elements insaid array—e.g. lenses—are arranged such that all sub-beams impinging onthe first reflecting means are directed towards the second reflectingmeans and not elsewhere e.g. back towards the array offenses. Thismeasure results in less wastage of the beam and furthermore avoidsdamage to other elements in the system by stray laser light. Such systemelements may include lenses, turning mirrors and even the laseritself—located “upstream” of the first and second reflecting means.

A third aspect of the present invention comprises the method of forminga nozzle in a nozzle plate for an ink jet printhead, the nozzle having anozzle inlet and a nozzle outlet in respective opposite faces of saidnozzle plate and a nozzle bore having an axis; the method comprising thesteps of:

-   -   directing a high energy beam towards said nozzle plate;        introducing divergence into said beam; and thereafter passing        said beam through beam converging means prior to impingement on        said nozzle plate, thereby to form a nozzle; wherein    -   the step of introducing divergence into said beam comprises        passing said beam through an array of optical elements to create        an array of sub-beams, each sub-beam having divergence, the        origin of divergence of each sub-beam lying apart from the        respective optical element; said array of sub-beams having a        greater width in a first direction than in a second direction        orthogonal to said first direction, said first and second        directions lying perpendicular to the direction of impingement        of said beam on said array; thereafter passing said array of        sub-beams through beam converging means prior to their        impingement on the nozzle plate, thereby to form said nozzle;        the angle of taper of the nozzle bore relative to the nozzle        axis in a direction corresponding to said first direction being        greater than the angle of taper of the nozzle bore in a        direction corresponding to said second direction.

The third aspect of the invention again shares the concept of splittinga high energy beam into sub-beams having an origin of divergence lyingapart from the plane of beam splitting and thereafter recombining thesub-beams through beam converging means. This third aspect alsocomprises an array of optical elements having a greater width in a firstdirection than in a second direction orthogonal to said first direction,which allows the production in a simple and accurate manner of nozzleshaving a greater taper angle in one direction than in another. This inturn yields a nozzle inlet having a greater dimension in one directionthan in the direction orthogonal thereto—such a configuration may beparticularly desirable where the ink supply channel to which the nozzleis attached is also non-axi-symmetric.

A fourth aspect of the present invention comprises a method of forming anozzle in a nozzle plate for an ink jet printhead, the nozzle having anozzle inlet and a nozzle outlet in respective opposite faces of saidnozzle plate, characterised by the steps of:

-   -   directing a high energy beam having a first axis extending in a        first direction towards said nozzle plate; directing said beam        at a first reflecting surface lying at an angle to said first        direction, said surface being arranged so as to reflect said        beam towards a second reflecting surface so arranged as to both        invert said beam and direct said beam along an axis collinear        with said first axis extending in a first direction; said first        and second surfaces being fixedly located relative to one        another, thereby to form an assembly, and rotating said assembly        about said first axis; said beam thereafter impinging on said        nozzle plate, thereby to form a nozzle.

As explained in greater detail in the description that follows, thistechnique results in a high energy beam having a uniform intensity at agiven radius and, when applied to the manufacture of nozzles, yieldsnozzle dimensions lying within tighter tolerance bands and consequentlya better quality nozzle.

A method of forming a nozzle in a nozzle plate for an inkjet printhead,the nozzle having a nozzle inlet and a nozzle outlet in respectiveopposite faces of the nozzle plate according to a fifth aspect of theinvention includes the step of directing a high energy beam at the faceof the nozzle plate in which said nozzle outlet is to be formed, wherebythe power of said high energy beam is initially held low and isincreased with increasing depth of the nozzle formed in said nozzleplate. As is also explained in greater detail in the descriptionhereafter, this technique gives a higher quality nozzle outlet, betterinternal finish and a more accurate nozzle shape.

The present invention also comprises apparatus for carrying out themethods outlined above.

The invention will now be described by way of example by reference tothe following diagrams, of which:

FIG. 1 is a schematic illustration of a first embodiment of the presentinvention when viewed in a direction X;

FIG. 2 is a view of the apparatus of FIG. 1 in a direction Y orthogonalto direction X;

FIG. 3 is a further embodiment of the present invention;

FIG. 4 a is a perspective view of yet another embodiment of the presentinvention; FIG. 4 b is a sectional view through the mirror arrangement82, 84 of FIG. 4 a;

FIG. 5 a is a sectional view through a beam conditioning deviceaccording to the present invention; FIG. 5 b is a schematic diagram ofthe beam section following conditioning;

FIGS. 6 a and 6 b illustrate the functioning of the device of FIG. 5 aat rotation angles of 0° and 90° respectively.

FIG. 1 shows an embodiment of apparatus for carrying out the methodaccording to one aspect of the present invention. Reference FIG. 20designates a nozzle plate in which a nozzle is to be formed. Theapparatus 10 comprises a source of a high energy beam such as a UVexcimer laser (not shown) which generates a high energy beam 30 which,after having undergone various beam conditioning processes (e.g.collimating, shaping of the beam to fit further optical devices located“downstream”), is directed at an array 40 of optical elements which, inthe present example, are cylindrical lenses 45. Such an array of lensesis commonly known as a flyseye lens.

The array 40 splits the beam into a corresponding array of sub-beams 50,each sub-beam having a focal point 52. As will be clear from the figure,after passing through the focal point 52, each sub-beam will bedivergent with a divergence angle (Aa, Ab in FIG. 1) and an origin ofdivergence at the focal point 52 of the respective lens 45 (note thatfor the sake of clarity, only outlines of those beams issuing from theoutermost lenses of the array 40 have been shown; the beams from lensesnearer the centre of the array will fall within these extremes). It willbe appreciated that range Aa, Ab of angles of divergence of eachsub-beam emanating from the lenses 45 will be much narrower than therange that would be expected from prior art techniques utilisingscattering. As shown in FIG. 1, the array of sub-beams issuing from thearray 40 is passed through a converging lens 60, thereby to recombinethe sub-beams at 56.

The recombined beam is directed at the aperture 72 of a mask 70, and tothis end, the mask is preferably located at a distance from the lens 60equal to the focal length of the lens.

Although in the example shown the focal point of the sub-beams 52 islocated downstream of the array 40, any arrangement where the focalpoint of the sub-beams is located before the subsequent mask 70 willsuffice: the lenses in the array 40 may for example diverge the incomingbeam such that the origin of divergence is located “upstream” of thearray 40, for example. The strength of the subsequent converging lens 60may be chosen such that the sub-beams still recombine.

As mentioned above and shown in FIGS. 1-3, the dimensions of the sectionof the recombined beam directly prior to impinging the plane of the maskare substantially equal to the dimensions of the aperture in said mask.The recombined beam passing through the aperture (and indicated by 74 inFIG. 1) is subsequently guided by means of a further convergent lens 80onto the surface 22 of the nozzle plate 20 where it ablates the materialof the nozzle plate, thereby forming a nozzle. The strength of the lens80 and the relative positions of the nozzle plate 20 and mask 70 arechosen such that an image of the mask aperture 72, illuminated by thebeam 56, is projected onto the surface 22 of the nozzle plate. Thenozzle section at the surface 22 and the mask aperture 72 can be seen tobe conjugate through the lens 80 and consequently, by changing the sizeof the aperture 72 the size of the hole formed in the surface 22 (whichforms the outlet orifice of the resulting nozzle) can be altered.

As is evident from the figure, the sub-beams 74 a, 74 b making up beam74 strike the surface 22 of the nozzle plate at an angle, with theresult that the section of the hole ablated by the beams increases withthe depth of the ablated hole. The resulting nozzle is thereforetapered, with the nozzle section at the “front” surface 22 of the nozzleplate 20 being determined by the mask aperture 72 and the section at the“rear” surface 24 being determined by both the aperture 72 and the angleof the incident beams.

The angle of the incident beams is determined both by the strength ofthe lens 80 and by the angles of divergence present in the beam 74passing through the aperture 72. The former preferably lies in therange: 0.4≦numerical aperture ≦0.65 (corresponding to magnification of×25 and ×52 respectively). The latter is determined by the strength ofthe lenses in the array 40 and also the geometry of the array. Asalready mentioned, the features whereby divergence is introduced intothe beam by splitting it into a number of sub-beams, each sub-beamhaving divergence, allows the angle of divergence of the nozzle formingbeam to be controlled that much more accurately. This in turn allowsaccurate control of the three-dimensional shape of the resulting nozzle,in particular its taper angle and the sections at the nozzle outlet andinlet.

Ensuring that the dimensions of the section of the recombined beamdirectly prior to impinging the plane of the mask are substantiallyequal to the dimensions of the aperture in the mask, as mentioned above,ensures that the high energy beam that finally impinges on the nozzleplate to form a nozzle does not suffer any significant reduction in itsdivergence—which might result in a corresponding reduction in nozzletaper. In practice, the section of the recombined beam will haveslightly greater dimensions than the mask aperture: were the recombinedbeam to be smaller than the mask aperture, then the mask would no longerplay any masking function and the image projected onto the front of thenozzle plate being not that of the aperture but that of the flyseyelens. It will also be evident from FIG. 1 that the matching between thedimensions of the aperture and the recombined beam also means that thedivergence angles (Ba, Bb in FIG. 1) of the sub-beams 74 a, 74 b makingup the recombined beam 74 at a position downstream of the mask 70correspond to the divergence angles Aa and Ab of the sub-beams 50upstream of the mask.

FIG. 2 is a view in a direction Y orthogonal to the direction of viewingX of FIG. 1 and illustrates the case where the array 40 has arectangular geometry, being wider in the X direction than in the Ydirection. It can be seen that the angle of divergence of the beamsleaving the aperture 72 is correspondingly greater than that shown inFIG. 1, as is the angle of taper of the nozzle in this direction andthus the dimension of the nozzle at the “rear” surface of the nozzleplate (indicated by x2 in FIG. 2 and greater than distance x1 in FIG.1). The overall shape of the nozzle at the rear surface will berectangular, in correspondence with the geometry of the array 40.

It should be noted that geometry of the array 40 can be altered eitherby rearranging the location of the lenses in the array or by blockingout some of the lenses of an existing array e.g. by means of a maskplaced directly upstream of the array.

The individual lenses making up the array 40 each Contribute a bundle ofdiverging beams, each bundle having a section which may be circular orsome other shape depending whether the optical elements making up thearray are lenses, prisms or otherwise having axi-symmetric or some othershape respectively. Whilst this feature is instrumental in obtainingmany of the advantages described in the present application, Itnevertheless results in the aforementioned section of the nozzle at the“rear” surface 24 having a corrugated outline. However, where this“rear” section is circular, the corrugations can be avoided by rotatingthe flyseye lens about its polar axis during the course of the nozzleforming process.

An alternative method of influencing the angle of the incident beams tocontrol nozzle taper is to interpose a further mask between the mask 70and the lens 80. Such an arrangement is illustrated in FIG. 3, thefurther mask being designated by reference FIG. 110, the correspondingaperture by 112. It is evident that the mask 110 blocks out those beamspassing through the aperture 72 which have divergence greater than acertain angle, resulting in a nozzle with reduced inlet size ×3. Thedimensions and shape of the further aperture can be varied to controlthe dimensions and shape of the shape of the nozzle at the rear surface,as is known from the aforementioned WO-A-93/15911.

Advantageously, a further converging (“field”) lens can be locateddirectly upstream of the mask aperture 72, as indicated by referenceFIG. 76 in FIG. 3. Movement of this lens in its own plane, i.e. parallelto the mask 70, allows the combined diverging sub-beams to be alignedwith the mask aperture. Nonalignment results in one side of the beambeing obscured more than another which in turn results in one side ofthe nozzle having a lesser taper than the other. Such asymmetry isundesirable in a nozzle.

According to another preferred embodiment of the invention, there islocated upstream of the flyseye lens a variable beam attenuator (notshown in the figures). Such devices are generally known in the art andfor this reason their construction will not be discussed here in anydetail. In the present invention, however, such a device isadvantageously employed to control the power of the high energy beamduring the nozzle formation process: at the beginning of the nozzleformation process, laser power Is held low to minimise damage to thenozzle outlet from exhaust products of the ablation process. Power isthen increased as the depth (and section) of the forming nozzleincreases. Towards the end of nozzle formation, high laser power isemployed to give the nozzle a good internal finish and to ensurefaithful reproduction of the shape of the nozzle forming beam. Theinitial rate of increase of laser power is preferably low, even zero,increasing once the forming nozzle has attained a certain depth.Measurement of the depth of the forming nozzle is not necessary: thepower of the laser may be controlled as a function of time, the timenecessary for a given process to reach a certain depth being readilydeterminable by experiment.

It will be apparent that many kinds of lens may be used for theconvergent lenses 60, 74 and 80 referred to above. However, it has beenfound particularly advantageous to use for the lens 80 a lens comprisingtwo mirrors of the type generally known as a Cassegrain reflective lens.An example is shown schematically in FIG. 4 a, the mask 70 andconvergent lens 60 having been omitted for the sake of clarity. FIG. 4 bshows the mirrors 82, 84 in section, from which is clear that themirrors are axi-symmetric, having reflective surfaces that are surfacesof revolution. Such a lens arrangement has a high magnification(equivalent to a high numerical aperture value), allowing a high degreeof angling of the incident beams relative to the axis of the lens(equivalent to a lower angle of incidence between beam and the surface22 of the nozzle plate 20) and the formation of nozzles of significanttaper. Such lenses also exhibit low aberration since the beam does notpass through any lens material but is simply reflected from one surfaceto another. Finally, it will be appreciated from FIG. 4 that thereflecting surfaces of such an arrangement are generally located awayfrom the surface of the nozzle plate and are thus less likely to becontaminated by debris generated during the nozzle formation process.

The flyseye lens may advantageously be adapted for use with a lens ofthe type described above by rendering the central lenses of the arrayinoperative e.g. by removing the lenses or blocking them out as shown inFIG. 4. Blocking may be achieved by means of a mask located directlyupstream or downstream. The sub-beams from these central lenses mightotherwise reflect back into and damage optical elements (even the laser)located upstream. In the embodiment shown, utilising an 6×6 array oflenses, the centre four lenses of the array are masked out.

FIG. 5 a shows apparatus that is particularly suited for use in themanufacture of nozzles for inkjet printheads and in particular for usewith arrangements described above. Located upstream of the flyseye lens,the device 120 comprises an assembly of three reflecting surfaces 121,122, 123 held fixed relative to one another by means of a housing 124 ithe assembly being rotatable together about an axis 125, for example inbearings 126 by means of a motor (not shown). The incoming beam 30 isdirected along the axis 125, strikes surface 121 and is reflected tosurface 122 and back to surface 123 whence it leaves the device, againalong the axis 125. In the example shown, the reflecting surfaces121,122,123 are high reflectance dielectric mirrors.

The paths of top and bottom sections (30 u, 301) of the beam atdifferent rotational angles of the device 120 are illustrated in FIGS. 6a and 6 b. When the device is at 0° rotation, as shown in FIG. 6 a,sections 30 u and 301 of the beam strike the reflecting surface 121 atdifferent locations along the axis 125 with the result that, followingfurther reflection by surfaces 122 and 123, the initially top and bottomsections 30 u and 301 exit the device at the bottom and top of the beamrespectively. However, with the device oriented at 90° as shown in FIG.6 b, both bottom and top sections of the beam strike the surface 121 atthe same axial location and no in version of beam sections 30 u and 301occurs. At 180° rotation of the device (not shown), sections 30 u and 30l will again strike surface 121 at different locations along the beamaxis with the result that inversion will take place.

It will therefore be evident that apparatus located downstream of therotating device 120 described above will be exposed to a beam 30′ havingan intensity at a given point P at a radius r from the beam axis thatvaries at a frequency corresponding to twice the angular velocity of thehousing 124 (see FIG. 5 b). Were the incoming beam 30 to be totallyhomogeneous, at least at a given radius r from the beam axis, the pointP would experience no change in beam intensity. In practice, however,the beam 30 generated by the laser is not homogeneous, even at a givenradius, with the result that the point P will experience a periodicallyvarying beam intensity. Such a varying intensity does nevertheless havethe virtue of having the same average value for all points irradiated bythe beam which are located at a radius r from the beam axis. Since beamintensity at a point translates into rate of material removal at thenozzle plate, use of the device described above results in nozzles thatare more uniform (at least at a given nozzle radius) than would beobtained using a beam not subject to such conditioning.

The use of discrete reflecting surfaces 121, 122 and 123 is particularlyappropriate in a device employing a high energy beam: these have theadvantage of low aberration when used with high energy beams, as wellhaving lower losses and being more robust than conventionallenses/prisms. In the example shown above, high reflectance dielectricmirrors are used.

It should be noted that other types of beam homogeniser, as are wellknown in the art, may be used in place of/in addition to the beamconditioning device just described.

A further imperfection in real-life optic systems is the presence ofstray beams caused by imperfections in the optical elements making upthe system: such stray beams, if allowed to hit the nozzle plate, mayresult in a nozzle that deviates from the ideal. This can be avoided bythe use of a spatial filter, shown by way of example in FIG. 3, andcomprising a mask 130 placed just in front of the nozzle plate at thepoint where the beams cross prior to impinging on the nozzle plate. Theaperture in the mask is chosen to pass the nozzle-forming beam yetexclude any stray beams failing outside of the nozzle-forming beam. Theaccuracy of the aperture is therefore crucial. Advantageously, theaperture can be formed by the in situ ablation of a mask blank using thesame beam and optics subsequently used for nozzle ablation. The materialof the mask blank should of course be chosen such that, unlike thenozzle plate material, it does not ablate significantly under the actionof stray beams.

A further process step for increasing the quality of the manufacturednozzles is to carry out the ablation process in an atmosphere of Heliumor Oxygen. Accordingly, the nozzle plate is placed in a chamber suppliedwith the appropriate gas and having a window through which the beam istransmitted. Components such as the spatial filter which lie very closeto the nozzle plate may also be accommodated in the chamber. Helium usedin the chamber acts as a cooling medium, condensing the ablationproducts before they have the opportunity to damage any other part ofthe nozzle plate, whilst oxygen used in the chamber reacts with theablation products, turning them to gas. Both methods result in a cleanerend product.

The present application is directed in the main to methods ofmanufacturing nozzles in a nozzle plate of an inkjet printhead. Althoughonly a single nozzle is shown in the figures, most designs of printheadwill have a substantial number of nozzles e.g. 64 or 128. Manufacturingtime can obviously be reduced by forming more that one nozzle at a time,these being either nozzles in the same printhead or nozzles belonging toseparate printheads. However, full optical systems of the type shown inFIGS. 1 and 2 are not necessarily required for each nozzle to be formedsimultaneously: for example, the beam from a single high energy beamsource may be used to feed a number of individual optical systems.Furthermore, only a single variable beam attenuator is necessary if itused to control the power of the single beam prior to splitting.Alternatively, the beam splitting optics may be inserted between themask 70 and the convergent lens 80, thus reducing duplication to theconvergent lens 80 and any other elements (spatial filter etc.) thatmight be required downstream thereof.

As regards the printhead itself, the nozzle plate 22 is made of amaterial, e.g. polyimide, polycarbonate, polyester, polyetheretherketoneor acrylic, that will ablate when irradiated by light from a UV excimerlaser. Whilst the process of ablation—which is well known in the contextof inkjet printheads as being capable of forming accurate nozzles—is tobe preferred, the present invention is not intended to be restricted tothis type of high energy beam. Radiation from other types of laser orother sources may be employed as a high energy beam.

It will be appreciated from the foregoing description that the presentinvention is particularly suited to forming tapered nozzles. In use, thebroad section of the tapered nozzle serves as the nozzle ink inlet andis connected to an ink channel of the printhead whilst the narrowsection of the nozzle serves as the droplet ejection outlet. The Ofront”surface of the nozzle plate in which the outlet Is formed may have a lowenergy, non-wetting coating to prevent ink build-up around the nozzles.In the case where this coating is applied to the nozzle plate beforenozzle formation, the beam must break through this coating as well asthe nozzle plate material.

Nozzles may be formed in the nozzle plate either before or afterattachment of the nozzle plate to the printhead (as is known in the art,see for example the aforementioned WO93/15911). In both cases, thelocation of the nozzle relative to the respective channel is importantand is facilitated by means for manipulating the nozzle plate/printheadrelative to the optical system prior to nozzle formation.

1-31. (canceled)
 32. A method of forming a nozzle in a nozzle plate foran inkjet printhead comprising the steps of forming a nozzle having anozzle outlet and of subsequently directing a finishing laser beam atthe nozzle to remove material by ablation and thereby provide aninternal finish.
 33. A method according to claim 32, wherein the step offorming the nozzle comprises the step of directing at the nozzle plate aforming laser beam having a power that is less than that of thefinishing laser beam.
 34. A method according to claim 32, wherein thenozzle tapers from nozzle inlet to nozzle outlet and wherein thefinishing laser beam impinges on that face of the nozzle plate in whichthe nozzle outlet is formed.
 35. A method according to claim 32, whereinthe step of directing a finishing laser beam at the nozzle occurs afterthe nozzle plate has been secured in an ink jet printhead.
 36. A methodaccording to claim 32, wherein an optical element in the path of thefinishing laser beam is rotated to cause a periodically varying beamintensity in the nozzle.
 37. A method according to claim 32, whereinbeam splitting optics are inserted in the path of the finishing laserbeam to enable more than one nozzle to be finished at the same time. 38.A method of finishing a nozzle formed in a nozzle plate for an inkjetprinthead, comprising directing a high power laser beam at the nozzle toremove material by ablation and thereby provide an internal finish. 39.A method according to claim 38, wherein the nozzle tapers from nozzleinlet to nozzle outlet and wherein the laser beam impinges on that faceof the nozzle plate in which the nozzle outlet is formed.
 40. A methodaccording to claim 38, wherein the step of directing a laser beam at thenozzle occurs after the nozzle plate has been secured in an ink jetprinthead.
 41. A method according to claim 38, wherein an opticalelement in the path of the laser beam is rotated to cause a periodicallyvarying beam intensity in the nozzle.
 42. A method according to claim38, wherein beam splitting optics are inserted in the path of the laserbeam to enable more than one nozzle to be finished at the same time. 43.A method of providing an internal finish to a nozzle for an ink jetprinter, comprising the steps of providing a nozzle, directing a laserbeam at a surface in which a nozzle outlet is formed and employing highlaser power thereby providing a good internal finish to the nozzle. 44.A method of conditioning an ink jet nozzle comprising: forming an inkjet nozzle, and directing a laser beam at an outlet side of the ink jetnozzle to internally finish and shape the ink jet nozzle.
 45. A methodaccording to claim 44, wherein the step of forming includes directing alaser beam having a first power at a side of a nozzle plate in which theoutlet of the ink jet nozzle is formed.
 46. A method according to claim45, wherein the step of directing further includes the laser beam havinga second power higher than the first power.
 47. A method according toclaim 44, wherein the step of forming includes first directing a firstlaser beam having a first power at a side of a nozzle plate to form asubstantial portion of the ink jet nozzle, and wherein the step ofdirecting includes directing the first laser beam to complete formationof the ink jet nozzle.
 48. A method according to claim 47, wherein thestep of directing includes increasing the power of the first laser beamto complete, internally finish, and shape the ink jet nozzle.
 49. Amethod of conditioning an ink jet nozzle comprising: forming an ink jetnozzle in a nozzle plate, and directing a laser beam at a side of thenozzle plate having an outlet of the ink jet nozzle to remove materialand internally finish the ink jet nozzle.
 50. A method according toclaim 49, wherein the step of directing includes removing material bylaser ablation at the outlet of the ink jet nozzle.
 51. A methodaccording to claim 49, wherein the step of forming includes directing afirst laser beam a the side of the nozzle plate to substantially formthe ink jet nozzle, and wherein the step of directing includesincreasing the power of the first laser beam.